Long-Distance Transmission of Power Underwater

ABSTRACT

A subsea long-distance power-transmission system comprises an electrically driven pumping station for producing a flow of pressurised working fluid and an electricity generating station having an electrical generator coupled to a fluid-powered machine. A supply duct extends across the seabed between the pumping station and the generating station, that duct being arranged to convey the flow of working fluid from the pumping station to power the machine. Electric power is supplied to the pumping station from an electric power source, such as a national power grid, and is supplied from the generator to an electric power consumer far distant from the power source, such as a subsea oil and gas installation.

This invention relates to the challenges of transferring or transmittingpower over a long distance underwater.

The invention may, for example, be applied to the supply of power froman onshore electric power source to an electric power consumer faroffshore, such as a subsea installation for the production of oil andgas. Conversely, the invention may also be applied to conveying powerfrom a source of electric power far offshore, such as an installationfor generating renewable energy, to an onshore electric power grid.

The invention may also be used to transfer power between offshorelocations that are far distant from each other. An example is from anoffshore platform to a remote subsea processing unit that receives powerfrom the platform to process hydrocarbons that then flow back to theplatform.

There is an increasing need to use, to generate and to transmitelectrical energy offshore and in particular at subsea locations. Thatneed has been driven by the growth in offshore renewable energygeneration and by an increasing requirement for high-powerelectrically-driven subsea equipment, particularly in the subsea oil andgas industry. An example of such equipment is a hydraulic power unit orHPU that is required to actuate remotely-operable hydraulically-drivenvalves or pumps installed on the seabed.

Previously, subsea valves or pumps have usually been used near offshoreoil and gas production platforms that provide a conveniently closesource of power. However, with the trend to exploit marginal subsea oiland gas fields, including remote or inaccessible fields, there is a needto minimise the cost of producing hydrocarbons from such fields. Oneapproach to this is to transfer at least some conventionally-topsideproduction and storage functions to a subsea location, hence displacingat least some hydrocarbon processing steps from topside to the seabed.

Moving processing steps from topside to a subsea location involvesplacing electrically-powered equipment not just underwater but alsoclose to wherever the subsea well may be. As a subsea well could be manykilometres away from a convenient source of electric power such as aplatform or a populated coastline, it follows that there is a need tosupply electric power at high voltage and/or high current over aninconveniently long distance underwater.

Various proposals have been made for long-distance transfer of electricpower. For example, WO 2013/077744 discloses high-voltage direct current(HVDC) transmission underwater. However, electrical cables that aresuitable for underwater use are extremely expensive. Such cables alsoneed to be manufactured in a single length, which involves an increasingrisk of detrimental defects arising over long distances.

WO 2017/044268 describes launching a guided surface wave across asea/air interface in order to transfer power from an offshore location.Power losses limit the maximum transmission distance that is achievableby that method.

Against this background, one aspect of the invention resides in a subsealong-distance power-transmission system. The system comprises: anelectrically-driven pumping station arranged to produce a flow ofpressurised working fluid; an electricity generating station having anelectrical generator coupled to a fluid-powered machine; and a supplyduct extending across the seabed between the pumping station and thegenerating station, that duct being arranged to convey the flow ofworking fluid from the pumping station to power the machine. The pumpingstation and/or the generating station may be situated underwater and maybe separated by a distance of 100 km or more.

The system may further comprise a remote electric power source that isconnected to the pumping station by a cable. For example, the electricpower source may be at an above-surface location. Similarly, the systemmay further comprise a subsea installation that is connected to thegenerating station to be powered electrically by energy conveyed alongthe supply duct by the flow of working fluid.

The system may be arranged as a closed loop, in which case the systemfurther comprises a return duct extending across the seabed between thegenerating station and the pumping station. The return duct is arrangedto convey the flow of working fluid from the generating station to thepumping station for re-pressurisation. In a closed-loop system, theworking fluid could, for example, be fresh water or monoethylene glycol(MEG).

Alternatively, the system may be arranged as an open loop. In that case,the working fluid is seawater, the pumping station comprises a seawaterinlet in fluid communication with one end of the supply duct and thegenerating station comprises a seawater outlet in fluid communicationwith an opposite end of the supply duct.

In both closed-loop and open-loop configurations, the system may furthercomprise a production flowline that extends along the supply duct andthat is in fluid communication with a subsea source of hydrocarbonproduction fluid. Conveniently, the production flowline may be disposedwithin and extend along the interior of the supply duct. A heater mayact on the working fluid in the supply duct. The production fluidsuitably flows in the production flowline in contra-flow to the workingfluid flowing in the supply duct.

The supply duct may comprise a penstock that has an accelerator portiontapering toward the generating station.

The generating station may, for example, comprise at least oneturbo-generator assembly that has: a hollow housing arranged to maintaina gas-filled space within the housing; at least one working fluid inletextending through the housing to effect fluid communication between thesupply duct and the gas-filled space; and a Pelton turbine supportedwithin the housing to turn in the gas-filled space in reaction to theflow of the working fluid entering the gas-filled space via the or eachworking fluid inlet.

The generator may conveniently be supported by the housing of theturbo-generator assembly. The housing may further comprise a chamber forreceiving the working fluid after the working fluid has impinged on theturbine. The housing may be penetrated by at least one working fluidoutlet that is in fluid communication with the pumping station or withthe surrounding seawater.

The inventive concept embraces a corresponding method of transmittingpower over a long distance underwater. The method comprises: supplyingelectric power from an electric power source to an electrically-drivenpumping station; using the electric power in the pumping station toproduce a flow of pressurised working fluid; conveying the flow ofworking fluid across the seabed from the pumping station to a machineremote from the pumping station; using the working fluid to power themachine; generating electric power in a generator driven by the machine;and supplying the electric power from the generator to an electric powerconsumer.

The method may comprise returning the flow of working fluid across theseabed from the machine to the pumping station, re-pressurising the flowand recirculating the flow back across the seabed to the machine in aclosed loop. Alternatively, the method may comprise drawing seawaterinto the pumping station from the surrounding sea to constitute the flowof working fluid and expelling the seawater into the surrounding seaafter using the seawater to power the machine.

A flow of hydrocarbon production fluid may be conveyed parallel to theflow of working fluid between the pumping station and the machine. Forexample, the flow of production fluid may be surrounded with the flow ofworking fluid. Where the flow of working fluid is heated, it isadvantageous for the flow of production fluid to be opposed in directionto the flow of working fluid.

The invention adopts an alternative approach to power transmission thatremoves the need for a long subsea electrical cable and for isolation orconnection of associated electrical systems. The invention reliesinstead on the pressure and flow of a working fluid, which may be wateror another liquid.

Embodiments of the invention provide a system for transferring electricpower over a long distance. The system comprises: anelectrically-powered pump; at least one conduit for transporting a fluidpressurised by the pump; and at least one turbine assembly at the otherend of the at least one conduit; wherein the turbine of the turbineassembly is rotated by the pressurised fluid and thereby produceselectricity. The length of the conduit may, for example, be more than100 km. This and other components of the system are preferably locatedsubstantially entirely underwater. For example, the electrically-poweredpump may be located on the seabed although its power supply could comefrom above the surface.

In some embodiments, the fluid system is a closed-loop system comprisinga higher-pressure supply fluid path from the pump to the turbine and alower-pressure return fluid path from the turbine to the pump. Forexample, the conduit may comprise a pipe-in-pipe pipeline, wherein aninner pipe contains pressurised fluid flowing to the turbine and anannulus or other space around the inner pipe contains return fluidflowing back to the pump, or vice-versa.

In a closed-loop system, the fluid is preferably fresh water or watertreated with additives to combat corrosion and microbial growth.However, the fluid could be another liquid such as MEG.

In other embodiments, the fluid system is an open-loop system in whichthe electrically-powered pump draws in and pressurises seawater abovethe prevailing hydrostatic pressure and the turbine rejects the seawaterback into the sea. In that case, the conduit may be a single waterpipeline, preferably with an inner diameter that is greater than 20 feet(6.1 metres).

A conduit for use in the invention may, for example, be a pipeline ofthermoplastic composite pipe, steel pipe, polymer-lined pipe or anycombination of such pipes. The conduit may contain at least one flowlinefor transporting a hydrocarbon production fluid. The conduit may containat least one heating element such as an electrical heating cable.

Thus, the invention provides an alternative to long-distance subseapower cables, using pipes or pipe-in-pipe structures to transport highpressure water out to an offshore location and there to generate powerremotely using a subsea turbine. Return water may, optionally, be pumpedback by suction of a nearshore pump.

Advantageously, the invention avoids having to convert from HVAC to HVDCand back to HVAC for subsea transport of power over long distances.Normally such a solution would require topside facilities.

In summary, a subsea long-distance power-transmission system of theinvention comprises an electrically-driven pumping station for producinga flow of pressurised working fluid and an electricity generatingstation that has an electrical generator coupled to a fluid-poweredmachine. A supply duct extends across the seabed between the pumpingstation and the generating station, that duct being arranged to conveythe flow of working fluid from the pumping station to power the machine.Electric power is supplied to the pumping station from an electric powersource, such as a national power grid, and is supplied from thegenerator to an electric power consumer far distant from the powersource, such as a subsea oil and gas installation.

In order that the invention may be more readily understood, referencewill now be made, by way of example, to the accompanying drawings inwhich:

FIG. 1 is a schematic side view of a first power-transmitting system ofthe invention;

FIG. 2 is a schematic side view of a second power-transmitting system ofthe invention;

FIG. 3 is a schematic sectional side view of a power transmitting linkfor use in the arrangements of FIGS. 1 and 2;

FIG. 4 is a schematic sectional side view of a variant of the powertransmitting link shown in FIG. 3;

FIG. 5 is a schematic sectional side view of a further variant of thepower transmitting link shown in FIG. 4;

FIG. 6 is a cross-sectional view through a conduit of the powertransmitting link on line VI-VI of FIG. 5; and

FIG. 7 is a cross-sectional view of a variant of the conduit shown inFIG. 6.

Referring firstly to FIG. 1 of the drawings, a first power transmittingsystem 10 of the invention transmits power from a source of electricpower 12 to a consumer of electric power that may be exemplified by asubsea oil and gas installation 14 on the seabed 16. The source 12 maybe a generating station or a node of a power grid.

In this example, the source 12 is on land 18 but it could instead bebeneath the surface 20, for example as part of an offshore renewableenergy installation. Conversely the consumer of electric power couldinstead be above the surface 20, for example a community on an islandthat is separated from the mainland by an expanse of sea.

The subsea installation 14 is far distant from the source 12, in theorder of several tens of kilometres away and potentially substantiallymore than 100 km away, for example 150 km to 300 km away.

In accordance with the invention, the power transmitting arrangement 10comprises a subsea power transmitting link 22 that, in this example,lies on the seabed 16. The link 22 conveys power along most, andpotentially nearly all, of the distance between the source 12 and thesubsea installation 14. At one end, close to the source 12, the link 22converts electrical energy from the source 12 into kinetic energy andpotential energy by creating a flow of working fluid at elevated workingpressure. The fluid flows along the link to drive a machine at the otherend of the link, close to the subsea installation. The machine drives agenerator to convert the energy conveyed by the fluid back intoelectrical energy to power the subsea installation 14.

Thus, at its end closest to the source 12, the link 22 comprises anelectrically-driven pumping station 24. Conversely, at its end closestto the subsea installation 14, the link 22 comprises a generatingstation 26 that in this example comprises a turbo-generator assembly. Atleast one fluid conduit 28 extends across the seabed 16 between thepumping station 24 and the generating station 26.

The pumping station 24 may comprise one or more pumps of any suitabletype, such as a positive-displacement pump, a centrifugal pump or anycombination of two or more pumps.

The or each pump of the pumping station 24 is driven by an integralelectric motor that is powered via a conventional electric power cable30 extending from the source 12 to the pumping station 24. To reduce itscost and to maximise its reliability, the cable 30 should be as short aspossible and hence only as long as is necessary to connect the pumpingstation 24 on the seabed 16 to the source 12 on land 18. As anon-limiting example, the cable 30 may be from 5 km to 15 km long.

A second, much shorter connector 32 connects the generating station 26to a power distribution system of the subsea installation 14.

FIG. 2 of the drawings shows a second power transmitting system 34 ofthe invention. This largely corresponds to the first power transmittingsystem 10 shown in FIG. 1 and therefore like numerals are used for likefeatures. The key difference between these variants is that FIG. 2depicts the source 12 as an offshore or inshore platform that has anon-board generating station for producing electric power. The umbilicalcable 30 that connects that source 12 to the pumping station 24 can bebeneficially shorter than the cable 30 shown in FIG. 1, potentiallybeing not much greater in length than the depth of the water between thesurface 20 and the seabed 16 at that location.

Moving on now to FIGS. 3, 4 and 5, these drawings show variants of thelink 22 that are denoted as 22A, 22B and 22C respectively. Specifically,FIGS. 3, 4 and 5 show further details of the generating station 26 andvariants of the fluid conduit 28 that extends between the pumpingstation 24 and the generating station 26. The generating station 26 islargely common to all of these embodiments and so will be describedfirst. The variants of the fluid conduit 28 shown in FIGS. 3, 4 and 5,denoted 28A, 28B and 28C respectively, will be described afterwards.

The turbo-generator assembly of the generating station 26 comprises ahollow, rigid, pressure-resistant and self-supporting domed shell orhousing 36. The housing 36 is rotationally symmetrical around asubstantially vertical central axis 38 and so is circular in plan view.

The housing 36 contains a generally toroidal manifold or ring duct 40for receiving high-pressure working fluid 42 from the fluid conduit 28.The ring duct 40 encircles the central axis 38. The housing 36 alsoencloses, and the ring duct 40 also surrounds, a Pelton turbine 44 thatis supported to spin about the central axis 38. Such a turbine 44 ischaracterised by an array of circumferentially-facing buckets that aredistributed angularly around the central axis 38.

The ring duct 40 supports, and is in fluid communication with, an arrayof nozzles 46 that face inwardly from the ring duct 40 and are spacedangularly from each other around the central axis 38. The nozzles 46 areoffset angularly from radial alignment with respect to the central axis38, all in the same circumferential direction. Thus, the nozzles 46 havetangential orientation to direct jets of high-pressure fluid from thering duct 40 into the buckets of the turbine 44 with substantialcircumferential or tangential momentum. The buckets reverse the flow ofthe jets to maximise the momentum change and hence the reaction forceapplied to the turbine 44.

The housing 36 is surmounted by, and supports the weight of, a generator48 such as an alternator. The generator 48 closes an open top of thehousing 36 and is coupled to the turbine 44 by a drive shaft 50 thatalso spins on the central axis 38.

A transformer may conveniently also be mounted on top of the housing 36,for example on top of the generator 48, but has been omitted from thesesimplified views. For example, the transformer could instead bepositioned elsewhere and connected to the generator 48 by cables orother conductors.

In the embodiments shown in FIGS. 3, 4 and 5, the working fluid 42 is aliquid such as water. Thus, the housing 36 extends downwardly below theturbine 44 to define an open-topped chamber 52 that serves as a drainagereceptacle for a liquid working fluid 42 that falls from the turbine 44under gravity after impinging on the buckets of the turbine 44. In thisrespect, a turbine of the Pelton type works most efficiently when itspins in a gas. Thus, advantageously, the surface of the liquid workingfluid 42 in the chamber 52 is kept beneath the turbine 44 by a pocket 54of air or other gas at elevated pressure that is trapped around theturbine 44 in the domed upper portion of the housing 36.

The ring duct 40 is in fluid communication with one or more supply ducts56 of the fluid conduit 28, through which the ring duct 40 receives aflow of high-pressure working fluid 42 that is expelled, in use, from anoutlet 58 of the pumping station 24.

In the examples shown in FIGS. 3, 4 and 5, the or each supply duct 56has a tubular penstock 60 that enters the housing 36 of the generatingstation 26. The penstock 60 comprises a frusto-conical venturi oraccelerator portion that tapers in a downstream direction to acceleratethe flow of liquid that enters the ring duct 40.

On entering the ring duct 40, the incoming flow accelerated by thepenstock 60 is deflected to follow the ring duct 40 in a circumferentialdirection corresponding to that of the jets projected by the nozzles 46.In consequence, a high-pressure, high-velocity flow of liquid impingesagainst the buckets of the turbine 44 and so drives the turbine 44efficiently.

The link 22A of the embodiment shown in FIG. 3 differs from the links22B, 22C of the embodiments shown in FIGS. 4 and 5 in that FIG. 3 showsa closed-loop system whereas FIGS. 4 and 5 show open-loop systems. Thus,the working fluid 42 in FIG. 3 is recirculated in the system rather thanbeing expelled into the sea and so could be fresh water treated withroutine additives, or another liquid such as MEG. The housing 36 of thegenerating station 26 is therefore closed so as to enclose the workingfluid 42 fully, apart from an outlet 62 in the housing 36 at the levelof the chamber 52 through which the working fluid 42 is returned to thepumping station 24.

Specifically, the conduit 28A shown in FIG. 3 comprises parallel supplyand return ducts 56, 64 that together extend between the pumping station24 and the generating station 26. As noted above, the supply duct 56effects fluid communication between the outlet 58 of the pumping station24 and the ring duct 40 of the generating station 26. Conversely, thereturn duct 64 effects fluid communication between the outlet 62 in thehousing 36 of the generating station 26 and a suction inlet 66 of thepumping station 24. Working fluid 42 flowing along the return duct 64that enters the suction inlet 66 of the pumping station 24 isre-pressurised by the pumping station 24 and recirculated through theoutlet 58 into the supply duct 56, as shown in FIG. 3.

Turning next to the links 22B, 22C shown in FIGS. 4 and 5, theseopen-loop systems have several features in common with the link 22Ashown in FIG. 3. Consequently, like numerals are used for like features.The main difference between FIGS. 4 and 5 and FIG. 3 is that there is nolonger a return duct 64, and hence there is no longer an outlet 62 inthe housing 36 of the generating station 26. Instead, the housing 36 hasan exhaust outlet 68 at the level of the chamber 52, through which aworking fluid 42 in the form of seawater is expelled into thesurrounding sea after passing through the turbine 44. The seawaterworking fluid 42 is drawn into the system through a suction inlet 70 ofthe pumping station 24 that now opens directly into the surrounding sea.The seawater working fluid 42 is pressurised by the pumping station 24and then flows via the supply duct 56 from the outlet 58 of the pumpingstation 24 to the ring duct 40 of the generating station 26.

The link 22C shown in FIG. 5 has all of the features of the link 22B ofFIG. 4, plus some additional features. Most notably, the supply duct 56,which is also shown in cross-section in FIG. 6, is widened toaccommodate a production flowline 72. A major portion of the productionflowline 72 extends along the interior of the supply duct 56 in parallelto the direction of flow of the seawater working fluid 42. In thisexample, that portion of the production flowline 72 extends centrallyalong the supply duct 56. Thus, in this case, the supply duct 56 is anannulus that surrounds the production flowline 72, which is thereforesubmerged in the seawater working fluid 42 that flows within the supplyduct 56.

As FIG. 5 shows, the direction of flow of hydrocarbon production fluidalong the production flowline 72 is typically opposed to the directionof flow of the seawater working fluid 42 along the supply duct 56. Thiscontra-flow arises as the production fluid flows from the subseainstallation 14 shown in FIGS. 1 and 2 back to the land 18 or to theplatform that houses the source 12 of electric power.

FIG. 5 also shows a heater 74 in the supply duct 56 upstream of theproduction flowline 72. The heater 74 heats the flow of seawater workingfluid 42 exiting the pumping station 24 for the purpose of flowassurance in the production flowline 72. The added heat beneficiallyreduces the thermal gradient between the seawater working fluid 42 andthe hotter production fluid in the production flowline 72. Theabovementioned contra-flow between the production fluid and the workingfluid 42 is also advantageous when using a heated working fluid 42. Inthat case, the temperature of the working fluid 42 will be at itshighest at the upstream end of the supply duct 56 coinciding with thedownstream end of the production flowline 72, where the production fluidwill tend to have lost more of its heat.

The cross-sectional view of the conduit 28C in FIG. 6 shows theproduction flowline 72 suspended on the central longitudinal axis 76 ofthe supply duct 56 by spacer elements 78 that extend radially outwardlyfrom the production flowline 72 to the inner surface of the supply duct56.

In this example, the supply duct is defined externally by a carrier pipe80 that comprises a polymer sleeve or outer pipe 82, for example of PVC,a steel inner pipe 84 and a concrete layer 86 in an annulus between theouter and inner pipes 82, 84. The concrete layer 86 adds weight tostabilise the supply duct 56 on the seabed 16 and also contributes somethermal insulation to retain heat in the seawater working fluid 42 thatflows within the supply duct 56.

By way of example, the outer pipe 82 may have an inner diameter of 60inches (1.52 m) the inner pipe 84 may have an inner diameter of 54inches (1.37 m) and the production flowline 72 may have an innerdiameter of 16 inches (40.6 cm).

Among variations within the inventive concept, a production flowlinelike that shown in the fluid conduit 28C of the open-loop system ofFIGS. 5 and 6 could be integrated with the fluid conduit 28A of aclosed-loop system like that shown in FIG. 3.

In this respect, FIG. 7 shows a fluid conduit 28D in which a bundle ofparallel pipes is encased within an outer carrier pipe 80 like thatshown in FIG. 6, hence comprising a polymer outer pipe 82, a steel innerpipe 84 and a concrete layer 86 in the annulus between the outer andinner pipes 82, 84. Again, the production flowline 72 is disposedcentrally in this example but it is now bundled with, and surrounded by,an array of pipes 88A, 88B for conveying working fluid 42 from thepumping station 24 to, and in an optional closed-loop system also backfrom, the generating station 26. The pipes 88A, 88B may be supportedwithin the carrier pipe 72 by transverse spacers or may themselves serveas spacers that support the production flowline 72. The pipes 88A, 88Bmay be of a polymer material such as PVC; the production flowline 72 istypically of steel but could be of a polymer composite material.

In this example, the pipes 88A, 88B comprise an equal mixture of supplypipes 88A for receiving high-pressure working fluid 42 from the pumpingstation 24 and return pipes 88B for returning working fluid 42 back tothe pumping station 24 at lower pressure after the working fluid 42supplied by the pumping station 24 has passed through the turbine 44.

Many other variations are possible within the inventive concept. Forexample, any of the various pipes of a conduit or a production flowlinecould be of steel or largely of polymers or of composite materials.Additional layers or components can be added to the pipes, such as aninternal liner or an outer coating. Such additional layers or componentsmay comprise polymer, metal or composite materials. Also, pipes can besingle-walled or of double-walled pipe-in-pipe (PiP) construction.

Other elongate elements such as auxiliary pipes and cables may beincluded in a conduit, extending in parallel with the other pipes of theconduit to carry fluids, power and data signals between the towheads.Longitudinally-distributed transverse spacers may hold the various pipesand other elongate elements of the bundle relative to each other

Foundations, fixings or anchors such as staples or pins may be spacedalong a conduit to support the conduit, to fix the conduit to the seabedand to prevent the conduit sinking excessively into the seabed.

Multiple pumping stations and/or heating units may be distributed alongthe length of the conduit, like repeater stations on a data transmissionline, to maintain the pressure and temperature of the working fluid.

The conduit is apt to be towed out to an installation site in multiplesections that are each a few kilometres in length. In this respect, themaximum length of each section may be constrained by the availability ofland at onshore fabrication facilities such as spoolbases or yards.However, a conduit can be made as long as required by fabricating itfrom multiple sections coupled end-to-end.

A towable conduit section can be prefabricated, assembled and testedonshore or in sheltered water before it is towed offshore forinstallation. Sections can be joined underwater or at the surface.Conveniently, multiple sections can be joined inshore at the surface,towed together to an installation site as a single unit and installed onthe seabed simultaneously in one operation.

Various towing methods may be used to transport conduit sections to anoffshore installation site. In particular, the sections may be towed atvarious depths in the water. Sections may be surface-towed at or near tothe surface, or near the seabed to protect them from the influence ofsurface water dynamics. Mid-water towing may be preferred, for exampleusing the controlled-depth towing method or CDTM as known in the art forinstalling pipeline bundles.

Stacking major components of the turbo-generator assembly along thevertical central axis simplifies installation and maintenance, allowingthe assembly as a whole, or any of its major components, to be loweredfrom or raised to the surface together or separately. Subsea-releasable,ROV-operable fastenings may be provided between the stacked componentsfor this purpose so that the assembly may be assembled or disassembledsubsea.

The or each supply duct could be provided with one or more valves thatare capable of controlling or blocking fluid flow. For example, one-wayvalves may allow water to enter the housing of the turbo-generatorassembly but block the egress of gas from the housing.

A Pelton turbine is preferred for its compactness and efficiency.However, in principle, the turbine could be different type of turbinesuch as a Francis turbine. It may also be possible to replace theturbine with a different machine to drive the generator, such as a screwexpander or other positive-displacement machine.

1-23. (canceled)
 24. A subsea long-distance power-transmission system,comprising: an electrically driven pumping station arranged to produce aflow of pressurised working fluid; an electricity generating stationhaving an electrical generator coupled to a fluid-powered machine, thegenerating station being situated underwater; a supply duct extendingacross the seabed between the pumping station and the generatingstation, that duct being arranged to convey the flow of working fluidfrom the pumping station to power the machine; and a subsea installationconnected to the generating station to be powered electrically by energyconveyed along the supply duct by the flow of working fluid; wherein thesystem is arranged as: a closed loop, and further comprises a returnduct extending across the seabed between the generating station and thepumping station, that duct being arranged to convey the flow of workingfluid from the generating station to the pumping station forre-pressurisation; or an open loop, wherein the working fluid isseawater, the pumping station comprises a seawater inlet in fluidcommunication with one end of the supply duct and the generating stationcomprises a seawater outlet in fluid communication with an opposite endof the supply duct to expel seawater into the surrounding sea afterusing the seawater to power the machine.
 25. The system of claim 24further comprising a remote electric power source that is connected tothe pumping station by a cable.
 26. The system of claim 25, wherein theelectric power source is at an above-surface location.
 27. The system ofclaim 24, wherein the working fluid is fresh water or monoethyleneglycol if the system is arranged as a closed loop.
 28. The system ofclaim 24, further comprising a production flowline that extends alongthe supply duct and is in fluid communication with a subsea source ofhydrocarbon production fluid.
 29. The system of claim 28, wherein theproduction flowline is disposed within the supply duct.
 30. The systemof claim 28, further comprising a heater acting on the working fluid inthe supply duct.
 31. The system of claim 28, wherein the productionfluid flows in the production flowline in contra-flow to the workingfluid flowing in the supply duct.
 32. The system of claim 24, whereinthe supply duct comprises a penstock that has an accelerator portiontapering toward the generating station.
 33. The system of claim 24,wherein the pumping station is situated underwater.
 34. The system ofclaim 24, wherein the generating station comprises at least oneturbo-generator assembly having: a hollow housing that is arranged tomaintain a gas-filled space within the housing; at least one workingfluid inlet extending through the housing to effect fluid communicationbetween the supply duct and the gas-filled space; and a Pelton turbinesupported within the housing to turn in the gas-filled space in reactionto the flow of the working fluid entering the gas-filled space via theor each working fluid inlet.
 35. The system of claim 34, wherein thegenerator is supported by the housing.
 36. The system of claim 34,wherein the housing further comprises a chamber for receiving theworking fluid after the working fluid has impinged on the turbine. 37.The system of claim 34, further comprising at least one working fluidoutlet extending through the housing in fluid communication with thepumping station.
 38. The system of claim 34, further comprising at leastone working fluid outlet extending through the housing in fluidcommunication with surrounding seawater.
 39. The system of claim 24,wherein the pumping station and the generating station are separated bya distance of at least 100 km.
 40. A method of transmitting power over along distance underwater, the method comprising: supplying electricpower from an electric power source to an electrically-driven pumpingstation; using the electric power in the pumping station to produce aflow of pressurised working fluid; conveying the flow of working fluidacross the seabed from the pumping station to a machine that is situatedunderwater and is remote from the pumping station; using the workingfluid to power the machine; generating electric power in a generatordriven by the machine; and supplying the electric power from thegenerator to an electric power consumer situated underwater; wherein themethod further comprises: returning the flow of working fluid across theseabed from the machine to the pumping station, re-pressurising the flowand recirculating the flow back across the seabed to the machine in aclosed loop; or drawing seawater into the pumping station from thesurrounding sea to constitute the flow of working fluid and expellingthe seawater into the surrounding sea after using the seawater to powerthe machine.
 41. The method of claim 40, wherein the electric powersource is at an above-surface location.
 42. The method of claim 40,wherein the pumping station is situated underwater.
 43. The method ofclaim 40, comprising conveying a flow of hydrocarbon production fluidparallel to the flow of working fluid between the pumping station andthe machine.
 44. The method of claim 43, comprising surrounding the flowof production fluid with the flow of working fluid.
 45. The method ofclaim 43, comprising heating the flow of working fluid.
 46. The methodof claim 43, wherein the flow of production fluid is opposed indirection to the flow of working fluid.